A variety of superplastic materials were developed in recent years. When stretched at appropriate strain rates at elevated temperatures, superplastic materials show significant elongation without local distortion or necking.
As to aluminum alloys, research works were concentrated on superplastic aluminum alloys having an elongation of at least 150% at elevated temperatures of 350.degree. C or higher. Conventional aluminum base superplastic materials include Al.78% Zn alloy, Al.33% Cu alloy, Al.6% Cu.0.4% Zr alloy (Supral), Al-Zn-Mg-Cu alloys (7475 and 7075 alloys according to the AA standard), and Al.2.5.6.0% Mg.0.05.0.6% Zr alloys. Such superplastic materials can be readily formed into complex shapes.
A number of attempts have been made to apply superplastic materials in a variety of uses by taking advantage of their improved forming ability at elevated temperatures.
In general, corrosion resistance considerations are essential in order that aluminum alloy materials be useful as interior and exterior building panels, containers, and cases (e.g., trunks). In this respect, aluminum alloy materials are most often subject to coating or anodizing prior to use. In the case of coating, aluminum alloy materials should have firm adhesion to coating films and good corrosion resistance after coating. In the latter case, aluminum alloy materials have to be prone to anodization and to become fully corrosion resistant after anodization. They are also required to be free of streaks or other irregular patterns after anodization in view of the outer appearance. For use as structural members, not only strength, fatigue resistance, and toughness after mechanical forming are required, but also improved adhesion and weldability are required since they are often attached to other members by adhesive bonding or welding. For use as interior and exterior building panels and cases (e.g., trunks), anodized aluminum alloy materials are desired to exhibit a placid grey or black color.
Conventional superplastic forming aluminum alloys contained a substantial amount of copper and similar alloying elements since superplastic behavior was of the main concern. As a consequence, they suffered from many problems.
(A) They were less corrosion resistant without anodization. PA1 (B) They were less amenable to anodization in that desmutting was poor and powdering occurred on the surface. PA1 (C) They were less corrosion resistant even after anodization. PA1 (D) After anodization, they often show streaks and other irregular patterns, and poor appearance therewith. PA1 (E) Adhesion and weldability are poor. PA1 (F) For coating application, it is rather difficult to pretreat the underlying surface for coating reception and thus the corrosion resistance after coating is low. PA1 (G) Cavitation often occurs with losses of strength, fatigue resistance and toughness.
The conventional superplastic forming aluminum alloys were improved in forming, but had many drawbacks including poor corrosion resistance as mentioned above. These drawbacks prevented the alloys from finding practical commercial use.
Also, in conventional superplastic forming aluminum alloys, no particular attention has been paid to their color after anodization. It was thus difficult to ensure that the anodized alloys consistently exhibited a placid grey or black color.